Like other joints of the body, the wrist joint is subject to severe arthritis that produces great pain, considerable deformity and substantial loss of function. Although many efforts have been made to develop a satisfactory prosthesis for the wrist joint, none has to date come into widespread use.
One approach to the design of a wrist joint prosthesis involves a ball and socket connection between a radial component and a metacarpal component. This approach provides virtually no restraint on any motion of the prosthetic joint--all restraint must, therefore, come from the remaining soft tissues of the joint, and such restraint is likely to be considerably impaired in the severely diseased joint.
U.S. Pat. No. 4,229,841 granted Oct. 28, 1980, for "Wrist Prosthesis" describes and shows a two-axes prosthesis in which extension and flexion are afforded by a hinge or pivot pin connection having a latero-medially oriented axis and lateral angulation is provided by a second pivot pin oriented in the dorsal-volar direction substantially perpendicular to the latero-medial axis. No provision is made for axial rotation. It is not clear from the specification or drawings of this patent how much restraint, if any, there is on the permitted motions.
The inability of the prosthesis of U.S. Pat. No. 4,229,841 to provide axial rotation is believed to be an important deficiency, perhaps not so much because of diminishment of the function of the reconstructed wrist joint but, rather, because the prosthesis is required to carry all loads due to forces tending to produce axial rotation of the prosthetic joint. These loads, especially in cases where they are imposed by abruptly acting forces, can lead to loosening of the bone-cement-metal fixation structures of the prosthesis.
U.S. Pat. No. 4,229,841 describes other proposed wrist joint prostheses and certain deficiencies the patentees discern in them. Suffice it for the present inventors to state that they believe the prior art designs do not fully meet the requirements of restoring as much normal function as possible, of ensuring as much stability as possible through restraint on all motions and of having as high endurance as possible to minimize failures due to loss of adequate bone-cement-prosthesis attachment.